The present invention relates to a method of producing magnetite particles that are useful for the production of toners.
Particulate magnetites that are produced by a precipitation method from aqueous solutions have long been known. The production of magnetite by the precipitation of iron(II) sulfate with an alkaline component and subsequent oxidation with air is known from U.S. Pat. No. 802,928 (Fireman, 1905). Starting from this basic invention, numerous patents have since been granted that relate to the production of magnetites by a precipitation method.
These materials were first used for the production of paints of all types. The particular advantage of magnetites compared with organic colorants and carbon black is that they exhibit a very much better resistance to weathering, so that paints of this type can also be used for external applications.
Precipitated magnetites are also used for coloring concrete moldings, such as concrete paving slabs and concrete roof tiles.
Magnetites have also been used for some time in electrophotography for the production of toners. Magnetites which are produced by a precipitation method are particularly preferred for the production of toners for copier equipment which employs single-component toners. The magnetic toners that are used for this purpose must have specific properties.
With the continuing development and improvement of copier equipment and printers, the requirements imposed on magnetic toners, and consequently on the magnetites that are used for this purpose, are becoming increasingly stringent. The most recent generation of printers achieves a resolution of more than 400 dpi (dots per inch), which has led to the development of finely divided toners with a very narrow particle size distribution.
As a consequence, the magnetites that are used for this purpose must have a very narrow particle size distribution. Moreover, a defined particle size is necessary, so that a homogeneous distribution of the magnetite particles in the finished toner is ensured. The magnetites themselves must have a sufficiently high electrical resistance in order to stabilize the latent image during the electrostatic transfer thereof.
In addition, the coercivity, saturation magnetization and above all the residual magnetization must be in the correct ratio to the prevailing field strengths in the machine. It is therefore necessary to develop toners, and thus to develop magnetites with special properties, for each class of copier equipment.
There is therefore a need for the production of magnetites that have properties that make them just as suitable for the production of magnetic toners as they are for use in colorants for coloring paper, plastics, lacquers, fibers, and concrete.
The production of magnetites by a precipitation method with the addition of silicon is described in JP-A 51/044,298 (Showa). Pure precipitated magnetites without the addition of extraneous elements can be produced batch-wise according to DE-A 3,209,469 or continuously according to DE-A 2,618,058. FeSO.sub.4 is used as the iron(II) salt in the aforementioned patents.
However, it is also possible to use any soluble iron(II) salt for the production of a magnetite by a precipitation method. The use of FeCl.sub.2, as described in DE-A 3,004,718, is particularly suitable for this application. The use of FeSO.sub.4 or FeCl.sub.2 has the advantage that both of these substances can be obtained very inexpensively as by-products of the steel processing industry. Apart from sodium hydroxide, which is the precipitant most frequently used, CaO or CaCO.sub.3 (DE-A 3,004,718), ammonia (DE-A 2,460,493), or Na.sub.2 CO.sub.3, MgCO.sub.3, or MgO (EP-A 187,331) can also be used as precipitants. Air is generally used as the oxidizing agent.
Methods of oxidation using nitrates have also been described, however (DD-A 216,040 and DD-A 284,478).
The production of Si-containing magnetites for use in magnetic toners is particularly advantageous. These exhibit a charging behavior that differs from that of pure magnetites and have a higher thermal stability at the same particle size. A method of producing particles of this type is described in JP-A 61/034,070 (Showa). The Si component here is added to iron(II) sulfate, which results in the precipitation of hydrated silica and thus results in a non-uniform distribution of the Si in the magnetite lattice.
U.S. Pat. No. 4,992,191 describes a magnetite comprising 0.1 to 5.0 atomic % Si with respect to Fe, which is claimed to be particularly suitable for the production of toners. In this respect, it is particularly important whether the Si is concentrated at the surface, as described in EP-A 808,801, or whether it is uniformly distributed over the particles as a whole. In addition, inorganic surface modifications are known from U.S. Pat. No. 5,688,852. Apart from Si, the elements Al, Zr, and Ti are used here in the form of their oxides or hydroxides. Methods of post-treatment with organic substances are also known, such as those which are described in EP-A 750,233, for example. Titanium-containing esters of long-chain fatty acids play a particularly important part here.
The particle size and particle shape of magnetites can be controlled by the pH during precipitation. At high values of pH and at correspondingly low values of the Fe(II)/NaOH ratio (less than 0.47), octahedra are obtained. These particles exhibit the highest coercivity and remanence. If magnetites are precipitated starting at an Fe(II)/NaOH ratio higher than 0.48, round particles distinguished by their very low remanence are increasingly obtained. Moreover, these particles are generally relatively finely divided by comparison with magnetites produced at other pH values. These relationships were identified by Kiyama in 1974 (Bull. Chem. Soc. Japan, 47(7) 1646-50 (1974)).
Another means of producing more finely divided magnetites is to reduce the precipitation temperature and reaction temperature. It is known from the aforementioned work of Kiyama that the range of existence of Fe.sub.3 O.sub.4 becomes smaller with decreasing temperature. According to this work, pure magnetite is formed at Fe(II)/NaOH ratios from 0.55 to 0.45 at a temperature of 50.degree. C. If the precipitation temperature and reaction temperature are increased, the range of existence of magnetite is broadened. Magnetite is obtained at higher temperatures, particularly in alkalis.
A further important factor of influence in the production of precipitated magnetites is the oxidizing agent. In the case of atmospheric oxygen, the efficiency depends on the distribution of air bubbles in the suspension. The tendency for the more thermodynamically stable goethite to be formed generally increases as the flow of air increases.
If the production of finely divided magnetites is desired, it is necessary to develop a method that takes all the aforementioned facts into account. It is also important that any additives used (e.g., Si or other metals) are capable of effecting a considerable shift in the tendency of magnetite to form.
The underlying object of the present invention was therefore to develop a method of producing magnetites that are particularly suitable for use in finely divided single-component toners. Apart from a small particle size (about 0.1 .mu.m), magnetites of this type must exhibit a low remanence, a low coercivity, and a sufficiently high thermal stability.
Furthermore, magnetites of this type must have a narrow particle size distribution.
This object has been achieved by the provision of the following method:
1. placing an alkaline component in a vessel and passing a protective gas through it, PA0 2. adding the silicate component, PA0 3. heating this mixture, with stirring, to the precipitation temperature, PA0 4. adding an iron(II) component, PA0 5. heating to the reaction temperature, and PA0 6. oxidizing with an oxidizing agent to obtain an Fe(III) content greater than 65 mol %. PA0 (a) placing an alkaline component in the form of an aqueous solution in a vessel under a protective gas, PA0 (b) adding 1.0 to 3.0 mol % (preferably 1.7 to 2.5 mol %), relative to Fe of the magnetite, of a silicate component to form a reaction mixture, PA0 (c) heating the reaction mixture to a precipitation temperature of 60 to 80.degree. C. (preferably 65 to 75.degree. C.), PA0 (d) adding an iron(II) component at a rate of 0.5 to 1.5 mol of Fe/hour per equivalent of the alkaline component until the pH of the suspension is 7.0 to 8.5 (preferably 7.2 to 7.6), and PA0 (e) oxidizing the suspension with an oxidizing agent at a rate of 20 to 5 mol % of Fe(II)/hour (preferably 14 to 10 mol % of Fe(II)/hour) to an Fe(III) content of 65 to 75 mol % of Fe(III).